I find this infrared anomaly super festinating.
There were cavities above all my window headers that never got insulated when I insulated the house 20+ years ago. What they looked like, and where they were located, became evident when I remodeled my bathroom recently.
I decided to drill one-inch holes all around under the soffits and insulate them. This required bringing my antiquated, rusted-out, insulation blowing machined back to life for one last job. A little duct-tape and it got us through the job–sad to see it at the dump.
This IR image shows where insulation was missing, with the sun hitting that side of the house in the evening . The second picture was taken in the morning, and you could see that the cool night air was pushing through the hole making a cold spot on the back side of the interior wall.
I never would have thought that air in that cavity could find a path to the interior of the house though lath and plaster. It was confirmed with the tissue paper over the hole, as did a smoke test.
It must be quite a tortured path for air movement. I think this accounts for the difficulty of air sealing old houses and evidenced by poor blower door tests.
The negative pressure was created by a big fan (on high) in a window on the opposite side of the house, that was being used to cool down the house over-night.
More fun with IR.
Charles Buell Consulting LLC